NSA tapping undersea fibers?

[Peter Fairbrother]

> Matt Crawford at crawdad at fnal.gov wrote:

>> Cable companies do this (from the surface) when they repair cables, but they
>> usually cut the cable before separately raising the cut ends and splicing in
>> a new section. I doubt that cable would be strong or extensible enough to
>> lift uncut, unless there was a lot of slack from eg a previous repair.
> 
> To lift the midpoint of a cable 1000 units long by 5 units requires
> only 0.067 units of slack, or the ability to stretch by 0.0067%.
> (This takes into account the catenary shape of the lifted cable.)

How did you calculate that? It's not obviously wrong, straight is
((((1000/2)+5)^2)^0.5)-500)/500 = 0.005%, but I'd be interested in your
assumptions about the catenary (as in "entia non sont multiplicanda praeter
neccessitatum', I don't reverse calculate catenaries unless I _really_ have
to).

Cable companies don't do it. I don't actually know why, but a few guesses:

a) cables get tangled on seabottom features at less than 1000 mile intervals

b) lifting a cable through obstructions might impose damaging bending
stresses, so they test the raised portion before doing anything else

c) put into human perspective, it's a string 3 feet long being lifted 1 cm
in the middle over a not-flat surface, and only touching the ground at the
ends. So:

d) the stress in the catenary is enormous. In view of general seabed
roughness that sort of catenary is impractical. to put it mildly. You are
talking about a few hundred miles of cable suspended closely above the
seabed. Cable sinks, deliberately, and thus has weight. depends of course on
the elasticity of the whole cable, but as the fibres are made of glass...

e) fiber optic cables can't take that much stress (or even that much strain)
and function properly

Just guesses. I might be wrong about the difficulty of raising an uncut
cable. Anyone know the real (limiting) reason cable companies don't do it?

-- Peter




---------------------------------------------------------------------
The Cryptography Mailing List
Unsubscribe by sending "unsubscribe cryptography" to majordomo at wasabisystems.com